1. Field of the Invention
The present invention relates generally to the field of pulverizers for grinding combustion fuels, such as coal, and, in particular, to a new and useful hydraulic loading system for the top grinding ring of ball and ring pulverizers, such as the type E and EL coal pulverizers made by The Babcock and Wilcox Company.
2. Description of the Related Art
Coal pulverizers are used to grind, dry and classify raw chunks of coal into fine solids which can be fluidized and fed, for example, to burners used in conjunction with industrial or utility boilers or furnaces. E and EL type pulverizers are ball and ring (or ball and race) type pulverizers which employ the ball-bearing principle to grind the coal. Presently known E and EL type pulverizers use a pair of vertical axis, horizontal grinding rings, that surround a number of balls arranged between the rings, to pulverize coal. The lower or bottom grinding ring rotates through connection to a rotating, vertical main shaft, while the upper or top grinding ring remains stationary and is spring loaded to create grinding pressure.
The coal is ground by contact with the upper and lower grinding rings and balls (collectively grinding elements). The lower and upper grinding rings are each provided with a race having a predefined, matching track contour that engages the balls. The force from the upper grinding ring pushes the balls against the coal layer on the lower grinding ring. The grinding rings and the balls are made of abrasion resistant alloys and comprise the major wear parts of the pulverizer. The balls wear evenly and remain spherical, since the balls are free to rotate in all directions within the confines of upper and lower grinding rings. Ground coal is swept from the grinding zone defined by the grinding rings and the balls by air for final particle size classification and subsequent pneumatic transport to one or more coal burners.
FIG. 1 shows a cross section of a BandW type EL pulverizer generally depicted as numeral 2. This type of pulverizer has a stationary top ring 4, one rotating bottom ring 6, and one set of balls 8 that comprise the grinding elements. The pulverizer includes a cylindrical housing 20. The pressure required for efficient grinding is obtained from externally adjustable, dual purpose springs 10. The bottom ring 6 is driven by the yoke 12 which is attached to a vertical main shaft assembly 14 of the pulverizer. The top ring 4 is loaded by the dual purpose springs 10 which provide the desired grinding pressure and hold the top ring 4 stationary. Raw coal is fed into the grinding zone where it mixes with partially ground coal that forms a circulating load. Pulverizer air causes the coal to circulate through the grinding elements where some of it is pulverized in each pass through the row of balls 8. As the coal becomes fine enough to be picked up by the air conveyed into the pulverizer around the perimeter of the bottom ring 6, it is carried to the classifier where coal of a desired fineness is separated from the stream of air and pulverized coal, and is carried out with the air. Oversized material is returned to the grinding zone.
The pulverizer is driven by bevel gears provided on the underside of the bottom ring 6 which engage corresponding bevel gear teeth on horizontal pinion shaft 16 provided in the base of the pulverizer 2. Both the vertical main shaft 14 and the horizontal pinion shaft 16 are mounted in roller bearings. Forced lubrication is provided for the entire gear drive by an oil pump submerged in the oil reservoir and gear-driven from the pinion shaft.
For further details of such EL type pulverizers, the reader is referred to Chapter 12 of Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright(copyright)1992, The Babcock and Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.
One of the main requirements for grinding coal in a pulverizer of this type is adequate loading on the grinding elements. EL pulverizer top grinding rings have historically been loaded, and horizontal/rotational movement restricted, by using up to six single coil, dual purpose springs 10. The springs 10 apply a predetermined grinding pressure, as dictated by the grindability of the fuel and fineness required, to the non-rotating top ring 4. The springs 10 also permit vertical movement of the top ring to compensate for variations in size of pieces of fuel and foreign material that pass through the grinding elements. Additionally, the springs 10 prevent rotation of the top ring 4, and, by eliminating rubbing of contact surfaces, thereby reduce pulverizer maintenance. As the grinding elements wear, the pressure is restored by adjusting screw-down bolts that pass through brackets attached to the top of the housing 20.
FIG. 2 illustrates a known mechanism for applying a load to the grinding balls 8 of a ball and ring pulverizer 2 as taught in U.S. Pat. No. 2,595,587 to Lester L. Leach and assigned to The Babcock and Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein. The mechanism has a set of spring loading assemblies 200 connected to the compression springs 10 for applying the load to the top grinding ring 4. Individual threaded rods 24 and captive nuts 58 in each of the spring loading assemblies 200 are used to adjust the load applied to each spring 10. The compression spring 10 of each spring loading assembly 200 is loaded individually by rotating the associated captive nut 58 to move the threaded rod 24 vertically, thereby increasing or decreasing the degree of load applied to each compression spring 10.
Maintaining the proper load on the grinding elements of ball and ring pulverizers is critical to their operation. As the balls and rings wear, the top ring 4 moves closer to the rotating bottom ring 6, and the loading spring compression relaxes, reducing the grinding force. The force reduction in turn causes a drop in the pulverizer capacity and coal fineness. With the mechanism described above, the pressure exerted upon the grinding elements can be suitably adjusted to compensate for the ring and ball wear without altering the restraining forces exerted by the springs 10 in a plane normal to the coil axes of springs 10. Each set of compression springs 10 and spring loading assemblies 200, however, must be individually loaded and calibrated with the remaining sets to ensure that an even loading is provided on the top grinding ring 4 and balls 8.
With conventional spring loading assemblies, the pulverizer must typically be shut down in order to reset the compression on the springs 10 and ensure even operation of the pulverizer. Large tools must be used to turn the captive nut 58 and adjust the loading in a time-consuming procedure. The procedure requires workers to be on top of the pulverizer to make some of the adjustments as well, creating a risk of injury to the worker.
It is an object of the present invention to provide an apparatus for rapid and uniform loading of the grinding balls used in a ball and ring coal pulverizer.
It is a further object of the invention to provide a hydraulic loading system having a central control for centrally applying a load to the top grinding ring of a coal pulverizer.
Yet another object of the invention is to provide a loading system for ball and ring pulverizers which accommodates wear of the grinding assembly without lengthy maintenance shutdowns.
Accordingly, a hydraulic loading system for applying a compressive load to the top grinding ring of a ball and ring coal pulverizer is provided. The hydraulic loading system replaces conventional, threaded spring bolt assemblies with linearly loaded, non-threaded spring bolt assemblies. A hydraulic cylinder and piston assembly is mounted above each spring. The high-pressure connections of all the hydraulic cylinder and piston assemblies are piped in parallel and joined at a common manifold. The low-pressure connections are arranged in the same manner using a separate manifold and piping. The hydraulic cylinder and piston assemblies are thus controlled simultaneously using a single hydraulic pressure source. The hydraulic cylinder and piston assemblies each drive the corresponding non-threaded spring bolt to compress the associated pulverizer spring, thereby adjusting the load on the top grinding ring which is applied to the rest of the pulverizer grinding components.
In one embodiment the hydraulic loading system comprises a hydraulic loading system for a ball and ring coal pulverizer for producing fine coal for combustion in a coal-fired furnace or boiler from a raw coal, the pulverizer having a housing, a stationary top grinding ring, a plurality of grinding balls and a rotating lower grinding ring, the hydraulic loading system for applying a uniform compressive force exerted on the top grinding ring and grinding balls against the lower grinding ring, the hydraulic loading system further comprising a plurality of hydraulic cylinder and piston assemblies mounted on the upper portion of the housing, a corresponding plurality of non-threaded rods, each rod connected to one of the pistons of the plurality of hydraulic cylinder and piston assemblies and passing through the upper portion of the housing, a corresponding plurality of compression springs, each compression spring connected, within the housing, between a corresponding non-threaded rod and the stationary top grinding ring, and control means connected to each of the plurality of hydraulic cylinder and piston assemblies for simultaneously generating the same pressure in each hydraulic cylinder and piston assembly for exerting the uniform compressive force on the top grinding ring.
In another embodiment the hydraulic loading system comprises a hydraulic loading system for a ball and ring coal pulverizer for producing fine coal for combustion in a coal-fired furnace or boiler from a raw coal, the pulverizer having a housing, a stationary top grinding ring, a plurality of grinding balls and a rotating lower grinding ring, the hydraulic loading system for applying a uniform compressive force exerted on the top grinding ring and grinding balls against the lower grinding ring, the hydraulic loading system comprising a plurality of non-threaded rods slidably inserted through an upper portion of the housing of the pulverizer, a corresponding plurality of compression springs, each of the plurality of compression springs connected between each of the plurality of non-threaded rods and the top grinding ring, and hydraulic means using hydraulic pressure for simultaneously and uniformly exerting a force on each of the plurality of non-threaded rods to compress each of the plurality of compression springs a uniform amount and exert the uniform compressive force against the top grinding ring, said hydraulic means mounted on the upper portion of said housing.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.